[syzbot] [io-uring?] [net?] WARNING: refcount bug in zcrx_unregister_user

[syzbot] [io-uring?] [net?] WARNING: refcount bug in zcrx_unregister_user

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    ea2bff00da89 Add linux-next specific files for 20260806
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10e27e49580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b2f9169913eb6d73
dashboard link: https://syzkaller.appspot.com/bug?extid=a43f81e0646c5bbd2b88
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=10d92132580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/473ce49245bc/disk-ea2bff00.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/4d5bfcd186f5/vmlinux-ea2bff00.xz
kernel image: https://storage.googleapis.com/syzbot-assets/281694927321/bzImage-ea2bff00.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+a43f81e0646c5bbd2b88@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: decrement hit 0; leaking memory.
WARNING: lib/refcount.c:31 at refcount_warn_saturate+0x70/0x110 lib/refcount.c:31, CPU#0: syz.0.17/5808
Modules linked in:
CPU: 0 UID: 0 PID: 5808 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
RIP: 0010:refcount_warn_saturate+0x70/0x110 lib/refcount.c:31
Code: c6 c0 fe 36 8f e8 90 e5 ee fc 83 fb 01 7e 22 83 fb 02 74 39 83 fb 03 74 47 83 fb 04 75 68 e8 f7 df ee fc 48 8d 3d b0 16 8b 0b <67> 48 0f b9 3a eb 66 85 db 74 3e 83 fb 01 75 4c e8 db df ee fc 48
RSP: 0018:ffffc900032d78c8 EFLAGS: 00010293
RAX: ffffffff84d8a429 RBX: 0000000000000004 RCX: ffff888032abddc0
RDX: 0000000000000000 RSI: ffffffff8f36fec0 RDI: ffffffff9063bae0
RBP: ffffc900032d7a10 R08: ffff888032abddc0 R09: 0000000000000005
R10: 0000000000000004 R11: 0000000000000000 R12: ffff8880353d2700
R13: dffffc0000000000 R14: ffff8880353d2744 R15: 0000000000000001
FS:  00005555674e4500(0000) GS:ffff888124f12000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000002020 CR3: 0000000079e90000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 __refcount_dec include/linux/refcount.h:461 [inline]
 refcount_dec include/linux/refcount.h:476 [inline]
 ref_tracker_free+0x706/0x830 lib/ref_tracker.c:303
 netdev_tracker_free include/linux/netdevice.h:4545 [inline]
 netdev_put include/linux/netdevice.h:4562 [inline]
 io_close_queue io_uring/zcrx.c:582 [inline]
 zcrx_unregister_user+0x2d8/0x430 io_uring/zcrx.c:681
 zcrx_unregister io_uring/zcrx.c:688 [inline]
 io_register_zcrx+0x875/0xc00 io_uring/zcrx.c:1021
 __io_uring_register io_uring/register.c:942 [inline]
 __do_sys_io_uring_register io_uring/register.c:1040 [inline]
 __se_sys_io_uring_register+0xc23/0x1ab0 io_uring/register.c:1017
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x166/0x510 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fa6a959e0d9
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffe56af7168 EFLAGS: 00000246 ORIG_RAX: 00000000000001ab
RAX: ffffffffffffffda RBX: 00007fa6a9825fa0 RCX: 00007fa6a959e0d9
RDX: 0000200000000100 RSI: 0000000000000020 RDI: 0000000000000003
RBP: 00007fa6a9635024 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fa6a9825fac R14: 00007fa6a9825fa0 R15: 00007fa6a9825fa0
 </TASK>
----------------
Code disassembly (best guess), 5 bytes skipped:
   0:	e8 90 e5 ee fc       	call   0xfceee595
   5:	83 fb 01             	cmp    $0x1,%ebx
   8:	7e 22                	jle    0x2c
   a:	83 fb 02             	cmp    $0x2,%ebx
   d:	74 39                	je     0x48
   f:	83 fb 03             	cmp    $0x3,%ebx
  12:	74 47                	je     0x5b
  14:	83 fb 04             	cmp    $0x4,%ebx
  17:	75 68                	jne    0x81
  19:	e8 f7 df ee fc       	call   0xfceee015
  1e:	48 8d 3d b0 16 8b 0b 	lea    0xb8b16b0(%rip),%rdi        # 0xb8b16d5
* 25:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2a:	eb 66                	jmp    0x92
  2c:	85 db                	test   %ebx,%ebx
  2e:	74 3e                	je     0x6e
  30:	83 fb 01             	cmp    $0x1,%ebx
  33:	75 4c                	jne    0x81
  35:	e8 db df ee fc       	call   0xfceee015
  3a:	48                   	rex.W


---
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Re: [syzbot] [io-uring?] [net?] WARNING: refcount bug in zcrx_unregister_user

[Edward Adam Davis]

#syz test

diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
index f1464ea8ca64..2dbddf80244d 100644
--- a/io_uring/zcrx.c
+++ b/io_uring/zcrx.c
@@ -567,11 +567,10 @@ static void io_close_queue(struct io_zcrx_ifq *ifq)
 		.mp_priv = ifq,
 	};
 
-	scoped_guard(mutex, &ifq->pp_lock) {
-		netdev = ifq->netdev;
-		netdev_tracker = ifq->netdev_tracker;
-		ifq->netdev = NULL;
-	}
+	guard(mutex)(&ifq->pp_lock);
+	netdev = ifq->netdev;
+	netdev_tracker = ifq->netdev_tracker;
+	ifq->netdev = NULL;
 
 	if (netdev) {
 		if (ifq->if_rxq != -1) {

Re: [syzbot] [io-uring?] [net?] WARNING: refcount bug in zcrx_unregister_user

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+a43f81e0646c5bbd2b88@syzkaller.appspotmail.com
Tested-by: syzbot+a43f81e0646c5bbd2b88@syzkaller.appspotmail.com

Tested on:

commit:         5e6de6a2 Add linux-next specific files for 20260811
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=138d9079580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=dd154b7aa6fe745e
dashboard link: https://syzkaller.appspot.com/bug?extid=a43f81e0646c5bbd2b88
compiler:       Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8
patch:          https://syzkaller.appspot.com/x/patch.diff?x=124c52c6580000

Note: testing is done by a robot and is best-effort only.